Electrically conductive refractory body

ABSTRACT

An electrically conductive refractory body suitable for use as a boat for the evaporation of metals consists essentially of TiB2, BN and TiH2. AlN may be optionally included.

Umted States Patent [191 [111 3,9

Passmore Dec. 23, 1975 ELECTRICALLY CONDUCTIVE [56] References CitedREFRACTORY BODY UNITED STATES PATENTS Inventor: Edmund Passmore,Wilmington, 3,544,486 12/1970 Passmore 252/518 x Mass.

[73] A i GTE s l i Incorporated, Primary ExaminerBenjamin R. PadgettDanvers, Mass Assistant Examiner-E. Suzanne Parr Filed: Nov. 1973Attorney, Agent, or firm-James Theodosopoulos [21] Appl; No.2 412,008 5ABSTRACT An electrically conductive refractory body suitable for 518 useas a boat for the evaporation of metals consists cs- [51] Int. Cl. HOlBl/00 semially f 13 BN and i AlN may be optionally [58] Field of Search252/518, 520; 106/55; included.

1 Claim, No Drawings THE INVENTION Electrically conductive boats are.commonly used in the vacuum deposition of metals, e.g. aluminum, ontosuitable substrates, such as paper or plastic film. Elecl tric currentis passed through the boat in order to heat it to a temperature at whichmetal will evaporate therefrom. Examples of materials commonly used inthe manufacture of such boats are graphite, composites of aluminumnitride and titanium boride, and composites of boron nitride andtitanium boride.

The useful life of such boats is generally quite short because of, amongother things, the high temperature of operation. For example, in thevacuum deposition of aluminum, the boats are resistively heated totemperatures of at least about 1450C. The combination of hightemperature, corrosiveness of the metal being evaporated and thermalcycling that may occur during the life of the boat can cause cracks tooccur in the boat. Such cracking is generally a major cause of failureof such boats.

It is a purpose of this invention to provide a composite boat that ismore resistant to cracking than prior art compositions.

A boat in accordance with this invention consists of a composition-oftitanium diboride, boron nitride and titanium dihydride, and may alsocontain aluminum nitride. Improved resistance to cracking results whenthe boat composition is within about the following ranges: to 65 volumepercent of EN; to 50 volume percent of TiB 1 to 5 volume percent ofTil-1 0 to volume percent of AlN.

In one example of a boat in accordance with this invention, 254 grams(41 volume of TiB 178 grams (57 volume of BN and 14 grams (2 volume ofTiH were thoroughly mixed together and then hot pressed in a vacuumchamber inside a graphite die mold at 2,050C and 4,000 psi for 4 hoursto yield a disc 4 3/16 inches in diameter by /a inch thick. The disc hada density of 3.06 grams/cc, which is 97.7% of the absolute theoreticaldensity of 3.235 grams/cc.

A similar disc was prepared of a prior art composition using the samehot pressing process, the composition consisting of 267 grams TiB 169grams BN and 9 grams of H BO Two evaporation boats having overalldimensions of 0.375 by 0.750 by 3.00 inches were machined from eachcomposition. Cavities were machined in the boats to hold the metalevaporant. The boats were then tested under identical conditions byself-resistance heating to the evaporation temperature range for A1(1,400" to 1,700C) in a vacuum chamber, and 0.060-inch diameter Al wirewas fed thereinto. 5 gms of A] were so evaporated at a rate of 1.0gm/minute, after which the rate was increased successively to 2.0, 3.0,4.0, 5.0, and 6.0 gms/min. with the evaporation of 50 gms at each rate.Thus, the test consisted of the evaporation of a total of 300 gms of A1at successively increasing rates from 1 to 6 gms/min. After the test,the boats were examined for evidence of cracking. In the two boats madein accordance with this invention, one had no cracks and the other hadonly very slight cracks. However, the two prior art boats were bothcracked to a substantial extent.

It is believed that the increased resistance to cracking may be due tothe reduced porosity, as shown by the higher percentage of theoreticaldensity, that results from the useofTiH The composition of thisinvention had a density that was 97.7% of theoretical absolute density,while the prior art boats density was only 94.5% of theoretical absolutedensity.

In order to determine whether TiH also increased the density ofcomposites containing AlN, a second pair of composites was made in asimilar manner to that described above. A composite in accordance withthis invention was made by mixing together 165 grams (31.5 volume TiB113.5 grams (43.2 volume BN, 88.5 grams (23.3 volume AlN and 12 grams (2volume TiH and hot pressing for 4 hours at 2,050C under 4,000 psipressure.

Simultaneously, a prior art composite consisting of 176 gms TiB 113.5gms BN, and 88.5 gms of AlN was also hot pressed under the sameconditions. Again, the composite of this invention had a significantlyhigher percentage of absolute theoretical density than the prior artcomposite, 97.0% versus 94.5%.

These tests show that the enhanced density conferred by the addition ofTil-I is attained whether or not AlN is included as a constituent of thecomposite.

In fact, the benefits conferred by the addition of TiH are also notlimited to composites containing TiB but are specifically applicable toany composite containing BN or to the latter alone. An example of theuse of TiH in the fabrication of BN is provided by the followingexample.

636 gms of a BN powder which had been previously shown to be incapableof consolidation into a sound, dense refractory body by hot pressing (orany other means) was mixed thoroughly with 328 gms of Til-I powder. Thiscomposition equals volume BN, 20 volume Til-I The mixed powders were hotpressed as described above at 2,050C and 2,500 psi for 4 hours into asound refractory body 5.03 inches diameter by 1.09 inch thick. Thedensity was determined to be 2.53 gms/cu cm, corresponding to 93% of theabsolute theoretical density of 2.72 gms/cu cm. Although the mechanismwhereby the TiH facilitates consolidation of the BN is not known, it issuggested that the following chemical reaction may be operative.

Attempts to make sound BN bodies using lesser TiH additions were notsuccessful. One of these comprised the equivalent of 10 vol TiH (halfthat of the above example) and exhibited extensive cracks normal to thehot pressing direction after simultaneous consolidation under the sameconditions used above. Crucibles were machined from both composites, andthe latter showed much thermal shock cracking in tests involvingexternal heating and evaporation of Al. In contrast, the former (20 volTil-l composite showed no cracking. A third composite comprising 5 volTil-1 was also made at the same time under the same conditions, andcompletely feel apart (disintegrated) after removal from the graphitedie. Thus, it is evident that the Til-I addition is the bonding agent,and that it must be present in sufficient amount (more than 10 vol TiHfor this grade of BN) in order to attain a sound body. However, lesseramounts of Til-I may be adequate to enhance densification with othertypes of BN powder, which are more capable of being densified by hotpressing because of better particle geometry or other reasons.

3 The above examples demonstrate the benefits conferred by the additionof Til-l its applicability to a wide range of composites containing BN,and indicate its usefulness in the manufacture of such composites. Itmay also be used in increasing the size of bodies which can be made witha specific unit of manufacturing equipment, as well as for enhancing thequality of such bodies.

0 to 30 volume percent of AlN.

1. AN ELECTRICALLY CONDUCTIVE REFRACTORY BOAT, FOR THE VACUUM DEPOSITIONOF METALS, CONSISTING ESSENTIALLY OF TIB2, BN AND TIH2, WHEREIN SAIDBOAT HAS THE FOLLOWING COMPOSITION: 5 TO 65 VOLUME PERCENT OF BN, 20 TO50 VOLUME PERCENT OF TIB2, 1 TO 5 VOLUME PERCENT OF TIH2, 0 TO 30 VOLUMEPERCENT OF ANLN.